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Farmers have been eager for a new generation of herbicide-resistant seeds because of the prevalence of weeds that have become immune to Monsanto's Roundup. The new genetically altered "Enlist" corn and soybeans developed by Dow AgroSciences would allow farmers to use the weed killer throughout the plants' lives.

But skeptics are concerned that use of the new seeds and the herbicide known as 2,4-D will lead to similar problems as weeds acquire resistance to that chemical as well.

"It's just so clear. You can see that you have this pesticide treadmill effect," said Bill Freese, a chemist with the Washington, D.C.-based Center For Food Safety, which promotes organic agriculture.

Most corn and soybeans grown in the U.S. are already genetically engineered, largely to resist Roundup, which was introduced in 1976. Before that, most farmers tilled their fields prior to planting, flipping the soil over and burying the weeds to kill them. The technique also exposed tilled earth to the air, creating problems with erosion and runoff.

Herbicide-resistant seeds permitted most farmers to stop tilling since they could spray fields after their plants emerged, killing the weeds and leaving their crops unharmed.

The new generation of plants "allowed us to do a better job of controlling the weeds, and therefore, we've been able to do a better job of preserving the soil, which is our primary natural resource," Ron Moore, who grows 2,000 acres of corn and soybeans in western Illinois, told The Associated Press.

The U.S. Department of Agriculture's plant-inspection agency concluded that the greatest risk from the new seeds developed by Dow AgroSciences was increased use of 2,4-D, which could accelerate the evolution of weeds resistant to it.

But, the agency said, resistance could develop anyway because 2,4-D is already the third most-used weed-killer in the nation.

Health risks

Freese and other advocates also raised concerns about possible health risks from increased use of 2,4-D and the chemical's tendency to drift beyond the area where it is sprayed, threatening neighboring crops and wild plants.

Dow AgroSciences has attempted to address that by developing a new version of 2,4-D and new equipment to use with it, company spokesman Garry Hamlin said.

The seeds and new 2,4-D have been approved in Canada but not yet sold there. The company has targeted their release in the U.S. for 2015, pending approval by various federal agencies. In anticipation of that, it has received import approval from seven nations and has applications pending in about six others to allow farmers who use the seeds sold under the Enlist brand to export their crops.

Some nations, particularly in Europe, have been resistant to genetically engineered crops, and consumer concerns have created a market for organic and other foods made without genetically modified ingredients.

Minneapolis-based General Mills announced Thursday that it had switched the sugar and cornstarch in original Cheerios to make that product GMO-free.

For now, Dow AgroSciences' seeds can only be used in tightly controlled trials.

The Center for Food Safety and the environmental group Earthjustice threatened legal action if restrictions are the seeds are lifted.

"This is among the worst applications of biotechnology," Andrew Kimbrell, executive director of the Center for Food Safety, told Reuters. "They will increase the use of toxic pesticides in industrial agriculture while providing absolutely no benefit to consumers."

Critics also point out that 2,4-D was one of the ingredients in Agent Orange, the Vietnam War defoliant blamed for numerous health problems suffered by soldiers and Vietnamese civilians during and after the war. Although the U.S. government blames the main health effects of Agent Orange on the other major component of the mixture and dioxin contamination, critics say 2,4-D has significant health risks of its own.

The Center for Food Safety said that 2,4-D and other herbicides of its class have been independently associated with deadly immune system cancers, Parkinson's disease, endocrine disruption and reproductive problems.

The public has 45 days to comment on the USDA report published Friday as part of the deregulation process. The Environmental Protection Agency is conducting a separate review on the impact of expanded use of 2,4-D, although it previously found the herbicide safe.